The process of traveling-wire EDM, also called wire-cut EDM, makes use of a continuous wire electrode (of a thickness of 0.05 to 0.5 mm) which is axially advanced and transported from a supply side through a machining zone to a takeup side. In the machining zone there is positioned a workpiece and an EDM gap is formed between the advancing wire electrode and the workpiece. A machining liquid, e.g. distilled water, is supplied to fill and flush the EDM gap while a series of electrical current pulses is applied between the wire electrode and the workpiece to produce a succession of electrical discharges through the liquid medium, thereby allowing material to be removed from the workpiece. As material removal proceeds, the workpiece carried on a worktable is displaced relative to the advancing wire electrode and generally transversely to the axial direction thereof, typically under numerical control along a prescribed path to form a desired cut in or on the workpiece.
In the path of wire travel, drive rollers driven by a motor are provided immediately upstream of the takeup side to apply a traction force to the wire to continuously feed it at a predetermined rate of advancement and allow it to be continuously withdrawn from the supply side and collected into the takeup side. Further, brake rollers driven by a motor and located immediately downstream of the supply side assures that the wire electrode will smoothly travel while being stretched under tension along the wire travel path. The wire travel path also includes a pair of guide members constituted typically by smooth arcuate bearing surfaces designed to change the direction of wire travel from the supply side to the machining zone and from the latter to the takeup side, respectively. These guide members may also be used or further an additional pair of guide or support members may be provided closer to the machining zones to serve as wire-positioning guides which precisely align the tightly stretched and traveling wire in a predetermined machining position across the machining zone of the workpiece.
With the aforementioned wire transportation and guiding arrangements, the traveling wire must be held to be positioned with an extreme precision between the support members across the machining zone. Because of an extremely small size of the machining gap spacing, say, 20 to 30 microns, it will be apparent that even a slight slip of the guided wire out of the predetermined position along the supporting surfaces would cause the wire in the machining zone to come in contact with the workpiece wall being cut, thus bringing about a short-circuiting which may result in a continuous arc discharge to the detriment of the machining stability and finish precision.
I have found that with the wire guide arrangement designed to insure utmost wire guiding precision, a wire positioning inaccuracy can nevertheless develop by reason of certain peculiar deformation of the wire caused in the machining zone and during the passage of the deformed wire along the guiding surface. In the traveling-wire electrical discharge machining of a workpiece, the wire electrode undergoes wear as well by electrical discharges and the wear, when excessive, may cause the breakage of the wire. Heretofore, in setting machining parameters and, inter alia, pulse parameters, much emphasis has therefore been placed on the conditions which could minimize the wire wear. I have observed, however that in traveling-wire EDM, the deformation action by successive electrical discharges on the traveling electrode takes place in a peculiar way not recognized heretofore. The electrode surface when struck by an electrical discharge has a discharge crater formed thereon with a crater rim raised from the crater depression. I have observed that the wire electrode departing the machining zone has generated successive peculiar raised formations apparently resulting from the successive or cumulative formation of discharge craters and each formation raised from the surrounding depressed floors has a built-up height which even protrudes considerably from the original diameter of the wire electrode. I have observed that these raised formations when passing through the machining positioning guide located immediately downstream of the machining zone result in a significant deviation of the wire in the latter zone from the preset machining position and hence result in a machining inaccuracy and instabilization which cannot be avoided by a precision design of the wire guide structure. In order to avoid the aforementioned raised formations, it is undesirable and impractical to employ a lower or greater speed of the travel of the wire through the machining zone since it has been found that on one hand a minimum rate of travel is required to hold a satisfactory mean machining current and on the other hand an increased rate of travel causes an excessive wear of the wire electrode which leads to its breakage.